This invention relates generally to filtering of electro-magnetic interference (EMI) and more particularly to the filtering of common-mode noise in products where noise couples into power lines.
As is known in the art any piece of equipment carrying an electric current is a possible source of electro-magnetic interference (EMI). Electronic devices may experience operating difficulties when subjected to EMI. In order to make electronic devices reliable and compliant with various standards it is desirable to reduce EMI to a minimum.
EMI can be viewed as any electromagnetic disturbance which results in the malfunctioning of an electronic device or interference to an electronic device. This disturbance may occur in three different ways namely through interference generation, interference transmission or interference susceptibility. EMI can be generated by varying electric or magnetic fields. The more abrupt the variation in energy flow, the broader the frequency range of the generated interference. Potential sources of interference are switches, relays, motors and transmitters. Once the interference is generated it may be transmitted from the source to an electronic device by free space radiation, inductive or magnetic field coupling, capacitive or electric field coupling, a common conductive or capacitive impedance, or any combination thereof. Susceptible electronic devices may be exposed to the generated interference by input conductors to the device.
One way of controlling EMI is to suppress the interference by use of a filter. It is desirable to provide the amount of suppression required with a minimum increase in the weight, bulk, space, complexity and cost of the assembly the filter will be part of. The purpose of filtering EMI is to attenuate by blocking or bypassing interference present on a line in order to prevent the interference from reaching the electronic devices. Filtering is often provided by introducing a high impedance into the path of the interfering currents, by shunting the interfering currents to ground through a low impedance, or a combination of both.
Common-mode noise is a type of EMI that is generated by sources such as fast switching of poorly terminated logic circuits, the oscillation of diodes of a power converter during transition, or poorly placed clock drivers. The common-mode characteristic of the noise means that the noise is passing through power lines in phase, with the return being the chassis of the enclosure housing the electronic components.
One technique used in the art to reduce common-mode noise is to surround the electrical conductors of the signal transmission path with a substance such as a ferrite material which can absorb undesired harmonics. As an example, for a single phase A.C. line application, the line and neutral conductors are wound bifilar (side by side) on a toroid. The resulting common-mode impedance serves to attenuate the common-mode noise traveling through the electrical conductors. This implementation however suffers from several disadvantages. The toroid has to be positioned on the electrical conductors between the power connector and the remainder of the circuit. Placing the toroid further away from the connector would result in the possibility of noise coupling into the electrical conductors making the design less effective, thus the toroid needs to be positioned close to the A.C. connector. The other disadvantage of this type of filter assembly is that at low frequencies the impedance of the filter is determined by the inductance of the toroid. Having a high number of turns is beneficial as the inductance is proportional to the square of the number of turns. Having situations where two or more turns are desired the required toroid becomes very large in order to handle two or more turns of the electrical conductors which are wrapped around the ferrite. The resulting toroid and wire loops take up valuable real estate inside the enclosure, as well as requiring an assembly process to wind the conductors on the toroid.